1. Field of the Invention
The present invention relates to a press-rolling process for producing a metal tubular product, in particular, a press-rolling process for producing a steel pipe from square billet by means of a press rolling piercer.
2. Description of the Prior Art
A press rolling piercer is, as described in Japanese patent publication No. 54-23675, an apparatus wherein a square billet is inserted into a pair of pressing rolls which are arranged opposite to each other in the vertical direction. A pusher applies pressure on the billet in the axial direction thereof and simultaneously, a plug is used to pierce the square billet.
In the rolling process using such a press-rolling piercer, if the materials are rolled within the mill capacity, no trouble occurs.
When an alloy of steel has a resistance to deformation beyond mill capacity, such as when special high alloy steel is rolled, the magnitude of the pusher pushing force, the rolling load, the plug thrust force and the torque etc. exceed the tolerance value of the mill capacity. In particular, the problem of the peak load, which appears when the front end of the billet is held by rolling mill rolls, is the most important.
In the rolling process using the piercer, the rolling operation progresses as illustrated in FIG. 1A to FIG. 1C. In the step illustrated in FIG. 1A, a steel billet 1, a pair of rolls 2, 2' and a plug 3 mounted on the end of a mandrel 4 are substantially and simultaneously contacted and then, the rolling operation progresses as illustrated in FIG. 1B. As a result, the plug 3 pushes out the billet metal situated at a substantially central position wherein the billet metal pushed out has the same volume as the plug. A part of the pushed billet metal flows in the longitudinal direction of the billet, another part flows in the transverse direction, and the remainder forms bulging at the entrance of piercing stage. This phenomenon is maintained even during steady rolling of the billet as illustrated in FIG. 1C.
In the step wherein a plug pushes out the billet metal as illustrated in FIG. 1B the thrust force of the plug continues to increase, reaches a peak load, and then is maintained at a little lower level than the peak load.
In a case where materials have a high resistance to deformation, such as when high alloy steel is pierced in a rolling mill designed for mild steel material, the peak load exceeds the capacity of the mill.